The new process has several advantages compared with the well-known common processes, which start from the fat itself by transesterification or from fatty acids, by esterification with glycerol. The chemical reaction for the glycerolysis of methyl ester ##STR1## is reversible and is governed by the Law of Mass Action. The use of excess glycerol and the simultaneous removal of the by-product methanol by vacuum are highly effective in displacing the equilibrium towards the right and hence to high monoglyceride yields. Hereby a `push-pull` effect will be forced upon the reaction. Fat glycerolysis--in contrast--is only `push` effective; although it responds to the use of excess glycerol, it does not respond to the application of vacuum except perhaps for the removal of traces of water, if necessary. Methyl ester as a class is far less hydrophobic than triglycerides like fats or fatty acids so that they can be much better emulsified in the glycerol, especially in the presence of monoglyceride. Moreover, a considerable thermal energy advantage and much less thermal degradation of the reaction components favour the methyl ester over fat glycerolysis; for example 130.degree. C. to 160.degree. C. for the methyl ester compared to about 250.degree. C. to 280.degree. C. for fat glycerolysis or the esterification of fatty acids with glycerol.
In general, there is a strong demand to more sophisticated industrial monoglycerides. Methyl esters provides here the opportunity to tailor-make the monoglyceride products to the exact end use requirements, since methyl esters can be fractionated at lower cost by distillation than fatty acids. On the other hand fat glycerolysis can only gain the distribution of the acyl groups in the end products naturally found in fat and oils.
The most important commercial products are glycerol monostearate, monooleate and monoricinoleate. But the vast majority of monoglycerides are applied in various mixtures in the food industry as emulsifiers, diet fat etc. However, there is a large field in technical applications. Due to their ability to form stable emulsions, monooleate is suitable as emulsifying component, for example in fine mechanical oils, as water displacing oils and in grinding and polishing pastes (1). Glycerol monostearate is a well-known plasticiser, lubricant and softener for the processing of technical plastics. These few examples illustrate the necessity to produce monoglyceride tailor made and not a mixture based on several saturated fatty acids or of mixtures of saturated and unsaturated fatty acids as produced by nature.
Equation 1 shows the simplified overall chemical reaction for the glycerolysis of methyl ester to monoglyceride. It is in the sense simplified, since it represents only the main predominant reaction and it does not show the formation of the 2-acyl or .beta.-monoglyceride which cannot be omitted and takes place at every temperature in the order of at least 5 to 8% of the total monoglycerides formed with an increasing tendency up to 20% at higher temperatures (in the order of 250.degree. C.). This phenomenon is due to the known differences in the relative chemical reactivity of primary and secondary hydroxyl groups in direct esterification and transesterification reactions. However, it is observed during the storage of monoglycerides at low temperatures--even in the solid stage--that the .beta.-monoglyceride mostly vanish in favour of the a-monoglyceride during storage.
In general, equation 1 suggests a trimolecular reaction, which is misleading. However, the reaction occurs stepwise, at least in a series of three steps represented by two successive equations with different overall reaction rates. Glycerol and methyl ester reacts initially in a relatively slow reaction depending on the amount of the `emulsifier` monoglyceride present in the reaction mixture. But unfortunately, there are further equilibrium reactions as shown below, namely to diglyceride and triglyceride: EQU Methyl ester+glycerol.fwdarw..rarw.monoglyceride+methanol EQU Methyl ester+monoglyceride.fwdarw..rarw.diglyceride+methanol EQU Methyl ester+diglyceride.fwdarw..rarw.triglyceride+methanol
Apart from these reactions others occur like the transformation of two molecules of monoglycerides forming one molecule of diglyceride and glycerol. Anyway, this reaction series shows qualitatively, that the formation of glycerides occur stepwise starting with the monoglyceride with a preference for the .alpha.-position, followed by the diglyceride with a higher content of 1, 2 diglyceride until finally the triglyceride is formed. In every reaction step methanol will be produced. The reaction rate from methyl ester to monoglyceride is much slower than the corresponding one for the transesterification of monoglyceride to diglyceride.
One reason for the different reaction velocities can be seen in the special emulsifying characteristics of the monoglyceride, as Wollman et al (2) for example have shown. Glycerol is neither in methyl ester nor in the glycerides completely soluble, so that always two liquid phases are present at reaction conditions. Monoglyceride is not completely miscible with methyl ester nor with glycerol, but it has due to its structure, a similarity to methyl ester as well as to glycerol and hence excellent emulsifying characteristics in respect of glycerol and methyl ester.
Consequently the mass transfer is best at a high content of monoglyceride leading to high reaction rates from mono- to diglyceride due to the lower mass transfer resistance than for the previous and the subsequent reaction steps. In order to move the reaction equilibrium to glycerides the methanol being formed has to be removed, if a large surplus of glycerol will be avoided.
The reactions are reversible in the range of mutual solubility and are subject to the Law of Mass Action. Hence an excess of glycerol over one mole theoretically required for monoglyceride results in the displacement of the equilibrium to higher monoglyceride concentrations. On the other hand, the excess of glycerol, which remains unaltered in the reaction mixture, must be removed afterwards and thus the Law of Mass Action dictates that the reaction will reverse itself. For this reason, catalyst are in general, but not exclusively, neutralized in order to take advantage of the substantially reduced reaction rates, both forward and backward, of the uncatalyzed system. Thus, at the peak of the monoglyceride formation and hence highest level of monoglyceride content in the reaction mixture, the catalyst must be completely neutralized and the crude reaction mixture cooled down during which time the solubility of the excess glycerol is decreased to such an extent, that some of this excess is thrown out of the solution and separates into two phases; a heavier, mostly glycerol and a lighter, predominantly glycerides containing phase (for further details see (3)).
In complete accord with the Law of Mass Action the whole reaction mixture reaches very rapidly a new equilibrium, especially at high temperatures. In general the content of diglyceride will increase on account mostly of monoglyceride. However, the rate of reversion is much slower than in the case of the catalyzed system. It must be the primary aim to remove the excess of glycerol fast enough and to cool down the reaction mixture quick enough, before a significant reversion can occur. Thus, the catalyst extraction or neutralization together with the removal of the excess glycerol and possibly methyl ester are the key steps for an economical production of monoglyceride. It should be pointed out, that a complete removal or destruction of the catalyst by neutralization is not automatically a guarantee for no outbreak of reversion or rearrangement, respectively, unless cooling and the removal of the surplus amount of glycerol and methyl ester are rapid.
The glycerolysis reaction can be catalyzed by alkaline catalyst like NaOH, KOH, LiOH and CA(OH).sub.2 or sodium salts of lower aliphatic alcohol such as methanol, ethanol, ter-butanol or triols like glycerol. Among those sodium methylate and glycerolate, NaOH or KOH are quite common. In the literature one can find several references to alkaline catalyst function as emulsifiers. These alkali catalyst can generate small quantities of soap from free fatty acids present at least in small quantities in most fats and oils, having in mind the well known emulsifying performance of soap containing mixtures of glycerol and fatty materials. Anyway, emulsification assists reactivity, since the two otherwise nearly immiscible reactants are brought into more intimate contact with one another. The line between the terms `catalyst` and `emulsifier` are rather vague and somehow indefinite in glycerolysis, so that it is difficult to distinguish between them.